1. Field of the Invention
In general, the present invention relates to environmental control systems for aircraft that have pressurized cabins. More particularly, the present invention relates to environmental control units that utilize secondary turbines to compress ambient air, wherein the secondary turbines are driven by the bleed air from an aircraft engine.
2. Prior Art Description
Low flying, relatively slow aircraft do not require sophisticated environmental control systems for the inside of the aircraft cabin. The quality of the air within the aircraft cabin can be adjusted by simply opening and closing vents or windows. However, many modern aircraft are designed to fly at high altitudes and at high speeds. Such aircraft require pressurized cabins, where the pressure within the aircraft is artificially maintained. If an aircraft cabin is pressurized, fresh ambient air cannot simply be vented into the pressurized cabin from outside the aircraft. Rather fresh air must be compressed to a pressure that matches that of the interior of the pressurized cabin so that the fresh air will flow into the pressurized cabin.
Aircraft that are designed to fly at high altitudes typically have jet engines or turboprop engines. Such turbine engines have compressors that can compress air to pressures above one hundred pounds per square inch. As the air is compressed, it is heated and may achieve temperatures of over five hundred degrees Fahrenheit at sea level. Fuel is then added to the compressed air and ignited in a separate combustion section of the aircraft engine.
Air can be bled from the compression chamber of the engine, prior to the compressed air becoming mixed with the fuel. By bleeding some air from the engine, a source of high temperature/high pressure air can be obtained. In early designs for aircraft environmental control systems, engine bleed air was directly used to feed air into a pressurized cabin. Such an environmental control system is exemplified by U.S. Pat. No. 3,537,510 to Rennenberg, entitled Pressure And Temperature Regulator For Aircraft Bleed Air System.
Since engine bleed air is typically at a high pressure and at a high temperature, sophisticated heat exchangers and pressure regulators must be used to condition the bleed air so it is at the correct temperature and pressure to be introduced into the cabin. Should a component fail, high temperature bleed air could directly flow into the pressurized cabin, thereby quickly overheating the cabin and requiring the aircraft to land for safety concerns and repairs.
Although engine bleed air can be used to directly heat the passenger cabin of an aircraft, the bleed air itself is never cooler than the cabin and cannot be used to cool the cabin. Rather, in order to cool the passenger cabin of an aircraft, the hot, high-pressure bleed air is used to turn a secondary turbine. The secondary turbine is used to provide pressurized air to a secondary vapor cycle air conditioning system. Such a system is disclosed in U.S. Pat. No. 7,305,842 to Schiff, entitled Environmental Control System And Method For An Aircraft.
Vapor cycle air conditioning systems are advantageous in private jets and other smaller aircraft. Such systems have electrically powered air conditioning compressors. This enables the cabin of the aircraft to be cooled on the tarmac using ground power. Such systems also provide an efficient way to control the relative humidity in the cabin as the air conditioning evaporator approaches near freezing temperatures.
In large commercial aircraft, the volume of cabin air that must be conditioned is much greater than that of smaller private aircraft. Accordingly, the air conditioning system must be larger. The mechanical requirements and weight of a vapor cycle air conditioning system, therefore, become impractical. Instead, commercial aircraft use air conditioning systems that utilize an expansion turbine. The function of the expansion turbine in such prior art systems is to remove energy from the bleed flow air after it has been cooled by a heat exchanger. This cools the bleed airflow to a temperature below that of the aircraft cabin. The cooled bleed air can then be used to cool the aircraft cabin. Such prior art systems, however, are complex and require sophisticated heat exchangers. These systems also use substantial volumes of bleed air from the aircraft engines. Furthermore, although the air is cooled, it still contains bleed air and the contaminants that come with bleed air.
An obvious problem associated with such prior art environmental control systems is that although the engine bleed air is used to run an air cooling system, the bleed air starts at a high temperature before it is cooled. Thus, if a cooling component fails in the environmental control system, engine bleed air is directly fed into the passenger cabin and the passenger cabin can quickly overheat.
Another problem associated with such prior art systems is that a large flow of engine bleed air is needed to drive the cooling system. Thus, the cooling system may work well when the aircraft is in flight and the engine is at cruise power. However, when the aircraft is taxiing on the ground and the engines are idling, the cooling system works poorly.
Yet another problem associated with many prior art environmental control systems is that they require large volumes of bleed air from the engines. This results in direct power losses from the engines since it starves the engines of the high pressure air needed during combustion. Excess removal of bleed air from an engine also results in higher engine operating temperatures and increased maintenance requirements.
A need therefore exists for an improved environmental control system for an aircraft that can provide both heated air and cooled air for a pressurized cabin without ever directing engine bleed air, with its contaminants, into the cabin. In this manner, even when the environmental control system fails, engine bleed air will not flow into the pressurized cabin and the pressurized cabin will not overheat.
Many aircraft environmental systems use turbines. Whenever a high speed, high temperature turbine is used in an aircraft, the turbine must be continuously lubricated. Furthermore, the engines of the aircraft must have constant lubrication. There are many additives that are used in aircraft lubricants. Of particular significance are a grouped call tricresyl phosphates and organophosphates. These additives are used for their anti-wear properties. However, these same chemicals are also used in pesticides. Even trace amounts of these chemicals can have harmful and cumulative effects on susceptible humans.
Dangerous byproducts from the engine bleed air can be avoided by never allowing the engine bleed air to enter the passenger cabin. However, preventing lubricants from entering an aircraft's current environmental control system from the aircraft engines or a turbine that is part of that system has proven highly difficult. FIG. 7 of prior cited U.S. Pat. No. 7,305,842 shows a system that draws lubrication fumes out of turbocharger. However, this system relies upon a venturi valve that receives ram air during flight. Accordingly, the system only draws fumes when the aircraft is in flight and the ram is open. When the aircraft is on the tarmac, or when the ram is closed, no suction is produced and the lubrication fumes are unabated.
A need therefore also exists for an improved environmental air conditioning system for larger aircraft that provides air with less contaminants than has previously been achievable, both while the aircraft is in flight and at idle. This need is also met by the present invention as described and claimed below.